1. Field of the Invention
The present invention relates to an acceleration sensor capable of detecting accelerations in three axial directions including an X-axis direction, a Y-axis direction, and a Z-axis direction which are perpendicular or substantially perpendicular to one another.
2. Description of the Related Art
FIG. 16 is a schematic perspective view illustrating an example of an acceleration sensor (see, for example, Japanese Unexamined Patent Application Publication No. 2002-296293). An acceleration sensor 40 shown in FIG. 16 includes a frame portion 41 and a columnar weight member 42 disposed at a central portion of the frame portion 41. X-axis-direction beam portions 43a and 43b extend in the X-axis direction from either end of the weight member 42 in the X-axis direction toward the frame portion 41. Y-axis-direction beam portions 44a and 44b extend in the Y-axis direction from either end of the weight member 42 in the Y-axis direction toward the frame portion 41. Four auxiliary weight members 45a to 45d are connected to the weight member 42. Resistance elements Rx1 to Rx4 and Rz1 to Rz4 are provided on the X-axis-direction beam portions 43a and 43b. Resistance elements Ry1 to Ry4 are provided on the Y-axis-direction beam portions 44a and 44b. 
In the acceleration sensor 40 shown in FIG. 16, the central axes of the X-axis-direction beam portions 43a and 43b are arranged on a single substantially straight line that passes through the central axis of the columnar weight member 42 and extends substantially in the X-axis direction. In addition, the central axes of the Y-axis-direction beam portions 44a and 44b are arranged on a single substantially straight line that passes through the central axis of the columnar weight member 42 and extends substantially in the Y-axis direction. Each of the X-axis-direction beam portions 43a and 43b and the Y-axis-direction beam portions 44a and 44b are flexible.
The resistance elements Rx1 and Rx2 are arranged in the X-axis direction on the X-axis-direction beam portion 43a. The resistance elements Rx3 and Rx4 are arranged in the X-axis direction on the X-axis-direction beam portion 43b. The resistance elements Ry1 and Ry2 are arranged in the Y-axis direction on the Y-axis-direction beam portion 44a. The resistance elements Ry3 and Ry4 are arranged in the Y-axis direction on the Y-axis-direction beam portion 44b. The resistance elements Rz1 and Rz2 are arranged in the X-axis direction on the X-axis-direction beam portion 43a. The resistance elements Rz3 and Rz4 are arranged in the X-axis direction on the X-axis-direction beam portion 43b. The electrical resistances of the resistance elements Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4 vary in accordance with stress changes caused in the beam portions 43a, 43b, 44a, and 44b when the beam portions 43a, 43b, 44a, and 44b are deflected.
Wirings defining bridge circuits shown in FIGS. 17A to 17C including the resistance elements Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4 are provided on the beam portions 43a, 43b, 44a, and 44b, and the frame portion 41. FIG. 17A shows a bridge circuit including the four resistance elements Rx1 to Rx4. FIG. 17B shows a bridge circuit including the four resistance elements Ry1 to Ry4. FIG. 17C shows a bridge circuit including the four resistance elements Rz1 to Rz4. Reference symbol Vcc shown in FIGS. 17A to 17C indicates a voltage source input that is connected to an external voltage source. Reference symbols Px1, Px2, Py1, Py2, Pz1, and Pz2 indicate voltage detection elements.
The weight member 42 and the auxiliary weight members 45a to 45d are arranged in a floating state such that they can be moved as the beam portions 43a, 43b, 44a, and 44b are deflected. For example, when a force is generated in the X-axis direction due to an acceleration in the X-axis direction and is applied to the weight member 42 and the auxiliary weight members 45a to 45d, the weight member 42 and the auxiliary weight members 45a to 45d are shifted in the X-axis direction due to the applied force. Similarly, when a force is generated in the Y-axis direction due to an acceleration in the Y-axis direction and is applied to the weight member 42 and the auxiliary weight members 45a to 45d, the weight member 42 and the auxiliary weight members 45a to 45d are shifted in the Y-axis direction due to the applied force. In addition, similarly, when a force is generated in the X-axis direction due to an acceleration in the Z-axis direction and is applied to the weight member 42 and the auxiliary weight members 45a to 45d, the weight member 42 and the auxiliary weight members 45a to 45d are shifted in the Z-axis direction due to the applied force. When the weight member 42 and the auxiliary weight members 45a to 45d are shifted as described above, the beam portions 43a, 43b, 44a, and 44b are deflected.
In the acceleration sensor 40, when the beam portions 43a, 43b, 44a, and 44b are deflected as described above and stresses are generated in the beam portions 43a, 43b, 44a, and 44, the resistances of the resistance elements Rx1 to Rx4, Ry1 to Ry4, Rz1 to Rz4 change. When the resistances of the resistance elements Rx1 to Rx4, Ry1 to Ry4, Rz1 to Rz4 change, the resistances of the four resistance elements included in each of the bridge circuits shown in FIGS. 17A to 17C become unbalanced. As a result, the accelerations in the X, Y, and Z axis directions can be detected.
For example, when an acceleration is applied in the X-axis direction, voltages output from the voltage detection elements Px1 and Px2 differ from each other in the bridge circuit shown in FIG. 17A. The amount of acceleration in the X-axis direction can be detected using the voltage difference. When an acceleration is applied in the Y-axis direction, voltages output from the voltage detection elements Py1 and Py2 differ from each other in the bridge circuit shown in FIG. 17B. The amount of acceleration in the Y-axis direction can be detected using the voltage difference. When an acceleration is applied in the Z-axis direction, voltages output from the voltage detection elements Pz1 and Pz2 differ from each other in the bridge circuit shown in FIG. 17C. The amount of acceleration in the Z-axis direction can be detected using the voltage difference.
However, in the structure of the acceleration sensor 40 shown in FIG. 16, the linear beam portions 43a, 43b, 44a, and 44b are respectively arranged on four sides of the weight member 42 so as to connect the weight member 42 to the frame portion 41. Therefore, when the frame portion 41 is distorted due to thermal stress, the beam portions 43a, 43b, 44a, and 44b are also distorted by the distortion of the frame portion 41. As a result, compressive or tensile stress is caused in the beam portions 43a, 43b, 44a, and 44b. 
More specifically, in the acceleration sensor 40, the resistance elements Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4 arranged to detect the accelerations are provided on the beam portions 43a, 43b, 44a, and 44b. Therefore, even when no acceleration is applied, if stresses are generated in the beam portions 43a, 43b, 44a, and 44b due to the distortion of the frame portion 41 caused by the thermal stress, the electrical resistances of the resistance elements Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4 will change. As a result, there is a risk that voltages equivalent to those output when accelerations are applied will be output from the bridge circuits shown in FIGS. 17S to 17V even when no acceleration is applied.
In addition, in the acceleration sensor 40, the resistance elements Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4 arranged to detect accelerations are provided on the beam portions 43a, 43b, 44a, and 44b extending from the weight member 42 in four directions. Therefore, the resistance elements are arranged at separate locations.
When the beam portions 43a, 43b, 44a, and 44b are made of silicon, the resistance elements Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4, which are piezoresistive elements, are formed by doping phosphorus (P) or boron (B) into the beam portions 43a, 43b, 44a, and 44b at locations at which the resistance elements are to be arranged. In this case, if the locations at which the resistance elements are to be arranged are separate from each other, it is difficult to dope phosphorus or boron uniformly at each location. Therefore, the doping concentration differs at each of the locations at which the resistance elements are arranged.
In such a case, in the acceleration sensor 40, it is difficult to obtain a balanced state between the resistances of the four resistance elements in each of the bridge circuits shown in FIGS. 17A to 17C. Therefore, the accuracy of acceleration detection cannot be sufficiently increased.